1. Field of the Invention
This invention relates to a developing device for developing an electrostatic image on an image bearing member such as a photosensitive member.
2. Related Background Art
In an image forming apparatus of the electrophotographic type, it has heretofore been practiced to form an electrostatic latent image on an image bearing member, and develop the latent image by a developing device. The developing device uses a developing sleeve of a non-magnetic metal containing therein a magnet roller for conveying a developer.
The surface of this developing sleeve is provided with suitable unevenness with a view to improve the conveyability and chargeability of the developer. A pipe of an inexpensive non-magnetic metal such as aluminum or an aluminum alloy is often used as the developing sleeve, but these are relatively soft metals (Vickers hardness Hv=70 to 120) and therefore, have the disadvantage that when the developing sleeve is used for a long period of time, the unevenness of the surface thereof is decreased or the uneven state is changed by the abrasion by the developer and images of good quality which have been obtained at the initial stage of use become unobtainable.
Describing this in detail with reference to the accompanying drawings, FIG. 7 is a cross-sectional view showing a developing sleeve heretofore used. The developing sleeve 6 is formed of aluminum which is a non-magnetic metal, and a magnet roller 7 as magnetic field producing means is contained in it. This developing sleeve 6 is disposed in a developing device for rotation about the magnet roller 7, while on the other hand, the magnet roller 7 is disposed against rotation.
FIG. 8 is a cross-sectional view of the surface portion of the developing sleeve 6, and unevenness 11 is provided on the surface of the developing sleeve 6 by blast treatment using glass beads. As regards the surface of the developing sleeve 6 having such unevenness 11, even though the surface roughness thereof is Rz=about 4.0 .mu.m at the early stage of use, it decreases to Rz=about 3.0 .mu.m after the development of about 10,000 sheets. As the result, the conveyability and chargeability of the developer are reduced to a level lower than the initially set level, and the image density obtained is reduced.
As a countermeasure for such a problem, it has been proposed to form a hard metal layer 12 of Cr or Cu, Ni and Cr or the like having hardness Hv=450 or so on the uneven surface of the developing sleeve 6, as shown in FIG. 9 (Japanese Laid-Open Patent Application No. 3-41485).
Such a developing sleeve as is provided with metal plating is high in the smoothness of the surface thereof and cannot sufficiently charge the developer or produces an irregular coat of the toner.
This problem will become more remarkable particularly if a magnetic positive toner (one-component positive magnetic developer) is used for development in a reverse developing system using an amorphous silicon drum as a photosensitive drum (image bearing member).
That is, the photosensitive drum of amorphous silicon is excellent in durability but low in charging ability, as compared with an OPC drum heretofore frequently used. Even if the surface of the drum is charged by suitable charging means to form an electrostatic latent image on the photosensitive drum, it is charged only to the order of +400 V to +450 V. Therefore, the latent image potential becomes shallow and the image density becomes low and thus, if the DC component of developing bias is set to a rather high level, toner charged to the positive also adheres to white portions on the photosensitive drum which are not latent image portions, and a so-called fogged image is remarkably created and the quality of image is reduced. So, if a toner enriched in magnetism is used to reduce the fogged image, there cannot be obtained a charging amount necessary to secure sufficient density, as described at the outset.